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MURDERERS AND SERIOUS SEXUAL OFFENDERS WITH A CRIMINAL HISTORY
3. Murderers and serious sexual offenders with a
criminal history
This chapter summarises the results from the case-control analysis relating to the
previous criminal history of murderers and serious sexual offenders. The results are
presented in the form of tables of relative risks, where relative risk scores greater than
one indicate the ways in which elements of the previous criminal history reveal a
greater likelihood of becoming a murderer (or a serious sexual offender). Conversely,
relative risk scores of less than one reveal a reduced likelihood of becoming a
murderer (or a serious sexual offender). The tables in this chapter show only the
variables which, at the five per cent level, significantly increase or decrease the
relative risk of murder or serious sexual assault 9 .
For variables which are ages, time periods in years, or counts of some kind (e.g. the
age at the previous conviction, the length of time from the first conviction to the
target conviction, or the number of sentencing occasions or previous offences), the
mean values for the cases and for the controls are displayed in the first two columns.
The variables which measure either the number of sentencing occasions or offences
in a specific age range are considered only for those who were older than the upper
point of that age range at the time of the target conviction.
For the remaining variables – the presence or absence of a conviction for a
particular indictable offence 10 or a custodial sentence at some point in the previous
criminal history – the percentages of cases and controls having this measure are
shown in the first two columns. For these measures, the study makes an operational
distinction between those which are frequent among the cases (20% or more of
murderers or serious sexual offenders), those which are not quite so frequent but still
quite sizeable (over 5% but fewer than 20%), and those which are comparatively rare
(between 2% and 5% of cases). In addition, the measures that are even more rare –
occurring in less than two per cent of cases – are highlighted in a fourth category.
These are considered in more detail in Chapter 6 on ‘unusual offences’.
The third column of figures indicates the p-value (since only the significant results
are given in this chapter, these are all less than 0.05). The relative risks are shown
in the fourth column, with ‘(0.01)’ or ‘(100)’ being displayed when there were no
cases or no controls (respectively) present in the sample to enable accurate
estimation of the relative risk.
17
9 The results for those convictions
for offences, etc., which were
found not to be significant can be
found on:
www.cas.lancs.ac.uk/prcu/murd
erssa/supplement.pdf.
10 An offence which may be tried
at a crown court. It excludes
less serious summary offences
which are tried only in a
magistrates’ court.